1. Field of the Invention
The present invention relates generally to apparatus for filling valve bags with a flowable particulate material until a precise preselected weight of such material is contained within the bag. More specifically, the present invention relates to a valve bag filling machine which advantageously combines features of an auger-type filling machine and an air flow-type filling machine for increased productivity and accuracy in filling valve bags.
Valve bags are of the type that comprise a top which is open at one end so as to be able to receive the filling spout of a filling machine. The principal advantage of such bags is that they close automatically when filled and removed from the machine so that no special sealing procedures are needed to close the bags at their top subsequent to their being filled.
2. Prior Art
Both auger-type bag filling machines and air flow-type bag filling machines are well-known in the art. Typically, manufacturers of products in the form of a flowable particulate material such as various chemical powders utilize one or the other type of bag filling machine to fill a valve bag with such material to a preselected weight. In carrying out the filling process there are two inherently conflicting requirements. One such requirement is to fill the bag as accurately as possible to the desired weight. The other requirement is to fill the bag as quickly as possible so that the bag filling process does not become too time consuming and costly in the overall process of delivering the particulate material to the marketplace.
Auger feed machines typically are capable of filling bags to a high degree of weight accuracy by virtue of the fact that the auger feed device is capable of maintaining a consistent and measurable flow rate for most products. Weight accuracy is achieved by reducing the delivery rate at the end of the filling cycle to most closely approach the precise desired weight without substantially overfilling the bag which would reduce the profit to the manufacturer. However, auger-type filling machines have an inherent problem in the delivery rate during the first ninety five percent or so of the filling cycle. More specifically, the auger delivery rate is typically relatively slow. The rate is a function of auger capacity, efficiency of loading and the bulk density of the product. Typically, in an attempt to maximize the delivery rate, augers are made large. However, this tends to reduce the accuracy because the flow stream during the final stages of filling at low speeds is not as consistent as desirable.
Air flow pressure packers on the other hand are traditionally faster filling machines than comparable auger packers. The air flow packer fluidizes the product and drives the fluidized product into the bag at higher rates than are normally attainable with auger fillers. The difference in filling rates between air flow packers and auger packers is particularly evident when bags are being filled with lighter density materials. The inherent problem with air flow packers relates to back pressure. More specifically, the back pressure in the bag affects the consistency of the flow stream. The delivery rate is a function of the pressure differential between the chamber pressure from which the material emanates and the bag pressure into which the material flows. As the bag becomes filled, the pressure in the bag builds as venting through the bag is restricted. Consequently, the flow rate begins to deteriorate near the end of the filling cycle. However, it is at this time that the consistency of the flow rate is required to achieve maximum accuracy for final weighing of the bag. Consequently, it is difficult to achieve extremely high accuracy on powdered products when utilizing air flow filling. In addition, certain powders create a considerable amount of dust towards the end of the filling cycle.
Consequently, it will be evident that manufacturers of flowable particulate material which are shipped in valve bags have had to make a difficult choice between accuracy and filling rate depending upon whether they selected an auger-type bag filling machine or an air flow-type bag filling machine.